In an effort to reduce solvent emissions including greenhouse gases, many industrial coating processes now employ waterborne paints and other waterborne coating systems containing greatly reduced amounts of Hazardous Air Pollutant (HAP) solvents and other Volatile Organic Compounds (VOCs). These coating systems are sometimes applied using a rotary electrostatic atomizer which flows the coating system material onto an electrostatically-charged rotating (viz., spinning) disk or bell, and slings droplets of the thus-charged coating material toward a grounded conductive substrate. A frequent concern in such systems is the need to maintain electrical isolation between the electrostatically-charged rotary atomizer and the coating system material supply. Electrical isolation may be provided or aided by routing the coating system material through a transfer block having a piston and a pair of electrically isolated supply cylinders, or by routing the material through a pair of electrically isolated reservoirs. In operation, metered amounts of the coating system material are alternately supplied to the atomizer from a transfer block supply cylinder or from a reservoir while the other supply cylinder or reservoir is being refilled.
Many industrial coating processes require frequent material changes, for example to change colors in otherwise similar coating materials, or to change coating materials such as changing from a primer to a topcoat. To carry out such material changes in electrostatic coating equipment, the transfer block or reservoirs in the coating equipment may be flushed with water or an organic solvent and dried with compressed air. The flushing step removes unused coating material from the transfer block or reservoir, and the drying step establishes a “voltage block” that discourages loss of electrical charge into the water or solvent supply line.
Cleaning lines are sometimes also connected directly to a rotary electrostatic atomizer. The rotary atomizer manufacturer may recommend that a nonpolar, nonflammable solvent (e.g., amyl acetate, methyl amyl acetate, mineral spirits, high flash naphtha, toluene or xylene) be used for cleaning, and that conductive solvents (e.g., acetone, diacetone, butyl alcohol, Butyl Cellosolve, methanol or monoethyl ether of diethylene glycol) not be employed. The atomizer manufacturer may also recommend that if a polar solvent is employed for cleaning, that doing so be followed by cleaning with a nonpolar solvent to remove conductive residue on the atomizer's surface.
The organic solvents used to clean rotary electrostatic atomizers may pose environmental or other hazards, may represent a waste disposal problem, and often are expensive. Rotary electrostatic atomizer manufacturers warn against using excessive amounts of such solvents, as the solvent may penetrate past the seals typically used to protect the air bearings and air turbines used in typical rotary electrostatic atomizers and may damage or contaminate these delicate parts.